fix disposing native menus / events

samples/ControlCatalog/MainWindow.xaml.cs

@@ -34,6 +34,7 @@ namespace ControlCatalog
             
             fileMenu.NeedsUpdate += (sender, e)=>
             {
+                fileMenu.Items.Clear();
                 fileMenu.Items.Add(new NativeMenuItem("Test 1"));
             };
 

src/Avalonia.Native/IAvnMenu.cs

@@ -47,6 +47,8 @@ namespace Avalonia.Native.Interop
 
             events.Initialise(menu);
 
+            menu._events = events;
+
             return menu;
         }